Method for changing and restoring the sensitivity characteristic of diluted penetrants



Jan. 24, 1967 J. R. ALBURGER 3,300,642

- METHOD FOR CHANGING AND RESTORING THE SENSITIVITY CHARACTERISTIC OFDILUTED PENETRANTS Filed Feb. 5, 1964 INVENTOR. cflmss4wueaze XrraemyyUnited States Patent 3,300,642 METHOD FOR CHANGING AND RESTORING THESENSITIVITY CHARACTERISTIC 0F DILUTED PENETRANTS James R. Albnrger, 5007Hillard Ave., La Canada, Calif. 91011 Filed Feb. 5, 1964, Ser. No.342,717 Claims. (Cl. 25071) This invention relates to the detection ofsurface flaws in various types of material, particularly metal, andparticularly to maintaining dimensional sensitivity in dilutedinspection penetrants.

Penetrant inspection processes depend on the formation of entrapments ofa penetrant material in surface flaws on parts being inspected. At thecompletion of the process, exuded flaw entrapments are observed as thinliquid films, being revealed by their color or fluorescence under blacklight. The ability of a given penetrant to detect small flaws depends onits so-called dimensional sensitivity, this being a function of theconcentration of indicator dye in the penetrant.

In my issued patent, No. 3,107,298, there is described the effect ofdimensional threshold of sensitivity, and it is shown how the thresholdof sensitivity for a given dye solution depends on its concentration.Heretofore, where a certain penetrant sensitivity level has been wanted,it has been necessary to establish an appropriate dye concentration inthe penetrant. In conventional penetrant processes, this dyeconcentration, of course, remains constant and provides a constantsensitivity characteristic for the penetrant.

In some inspection applications where very large tanks of penetrant areemployed, it might be a natural desire to extend the penetrant processmaterial by use of an inexpensive diluent. However, such steps have notbeen given serious consideration in the past for the reason that anydilution of the penetrant would alter the concentration of the dye, and,therefore, alter the dimensional sensitivity of the penetrant. On theother hand, dilution of the penetrant material could result insubstantial cost saving.

The principal object of the invention, therefore, is to provide a methodof and system for utilizing diluted penetrant inspection materials.

Another object of the invention is to provide an improved method ofmaintaining a desired dimensional sensitivity in the penetrant processeswhich utilize diluted penetrants.

A further object of the invention is to reduce the cost of inspectionprocesses using flaw detection penetrants.

A better understanding of this invention may be had from the followingdetailed description when read in conjunction with the accompanyingdrawing in which a chart of two typical dilution curves is shown in thefigure:

Referring, now, to the figure, the curves A and B show the behavior ofdimensional thresholds of two typical penetrants as functions of theirindicator dye concentrations. While various dye solutions exhibitdiffering specific dilution characteristics, all of such characteristicshave the same general form in that the dimensional threshold of the dyesolution is shifted upward by dilution.

It has been discovered that if a given non-volatile penetrant is dilutedwith a volatile solvent material, the dimensional sensitivitycharacteristic of the penetrant may be made to move upward insensitivity along the dilution curve by a process of evaporation of thevolatile diluent. In this way, a penetrant which is formulated to yielda dimensional threshold of 250 millimicrons, for example, may be dilutedwith a volatile solvent to a point where the dimensional threshold isgreater than 2000 millimicrons, a sensitivity characteristic which isquite useless for the detection of surface flaws in parts. However, ifthe volatile solvent is allowed to evaporate from the mixture, thesensitivity threshold moves along the dilution curve back to theoriginal value of 250 millimicrons.

A wide variety of conventional penetrant materials may be utilized inthe method of this invention. Exemplary of such penetrant materials arethose which are set forth in US. Patents Nos. 2,764,556; 2,871,697;2,920,203; 2,953,530; 3,028,338; and in my copending application Ser.No. 492,676, originally filed as Ser, No. 254,487 on January 28, 1963,now abandoned. All of these prior art penetrant compositions, and infact all presently used penetrant materials, be they water-washable orpost-emulsifiable types, consist essentially of a relatively nonvolatileliquid vehicle in which there is dissolved a suitable fluorescent orvisible color sensitizer dye. As I have pointed out in my now issuedpatent No. 3,107,298, the flaw detection capability or sensitivity of agiven penetrant depends in part on the concentration of the sensitizerdye. Thus, any alteration in the effective concentration of an indicatordye in a given penetrant composi tion will result in an alteration ofthe effective flaw detection sensitivity.

As will be noted from the examples to be given below, the method stepsof this invention may be carried out using any type or kind ofinspection penetrant material and with virtually any relatively volatilesolvent liquid which is compatible with the penetrant employed. Forexample, oil soluble or so-called oil-phase penetrants may be dilutedwith volatile solvents such as chlorinated hydrocarbons. On the otherhand, many other kinds of volatile solvents may be employed inconjunction with appropriate compatible pentrants, one useful andinexpensive solvent being water, which may be used to dilute certainwater-extendible penetrants.

It should be noted that certain diluents have been employed at times inprior art penetrants. For example, water has been utilized as a diluentfor certain watermiscible glycol-type penetrant compositions. However,such materials have not previously been employed with the combination ofmethod steps including dilution of the penetrant and subsequent exposureof test parts coated with the diluted penetrant so as to produce anevaporation or drying of the volatile diluent and a restoration of thesensitivity characteristic of the penetrant to a predetermined value.

In general, the prior art penetrants which have utilized water as adiluent are employed in a thus-diluted manner for the reason that thepresence of water acts to inhibit an unwanted deleterious explosivereaction when the penetrant is used on parts which are to be wetted withliquid oxygen.

The present invention contemplates the use of a relatively volatilediluent which will remain as a relatively stable extender for aninspection penetrant, but which will evaporate to completion, if exposedto draining and drying, during the period of time required forpenetration of the inspection penetrant liquid into surface flaws. Suchsolvents should be distinguished from aerosol propellant liquids whichare extremely volatile and which do not remain with the penetrant for anappreciable time while in contact with test surfaces. Following thefinal step of exposure and drying of coated test surfaces to produceevaporation of volatile diluent material, as hereinafter described,subsequent process steps, such as emulsification, washing, development,and inspection may be carried out in accordance with conventionalmethods.

Referring, now, to the chart of the figure, curve A is a sensitivitycurve of a conventional non-volatile fluorescent, oily penetrant havinga dimensional threshold of fluorescence of 100 millimicrons at anequivalent dye concentration of grams per pint which has been dilutedwith v3 perchloroethylene in the proportion of 40 partsperchloroethylene to one part of penetrant. The initial sensitivity ofthis penetrant is illustrated at point 6 on the curve A in the figure.The diluted mixture has a dimensional threshold of fluorescencecorresponding to point 7 or about 630 millimicrons.

With this penetrant, the parts being inspected are dipped in an opentank containing this diluted penetrant and are suspended over the tankto drain and dry. During a drain-dry period of about ten minutes, thevolatile perchloroethylene evaporates from the film of liquid from theparts, whereupon the dimensional sensitivity of the liquid film isrestored to its original value of 100 millimicrons. From an economicalstandpoint, the diluted penetrant might cost approximately $15.00 pergallon, whereas the perchloroethylene might cost about $2.00 per gallon.So with the diluted mixture costing about $2.32 per gallon, a costsaving for the tank of material would be of the order of 84.5%. Inaddition to the saving in cost of the material installation, anadditional saving is effected by virtue of the fact that theperchloroethylene, being a very heavy vapor, pours back into the tankwhere it remains. Replenishment of the tank is accomplished by theaddition of an appropriate amount of undiluted penetrant. v

Referring to curve B, a non-volatile water-miscible visible dyepenetrant having a dimensional threshold of color of 500 millimicrons ata dye concentration of 10 grams per pint, is diluted with water in theproportion of 10 parts water to one part penetrant. The initial sensitivity of the penetrant is illustrated by point 11 on the dilution curveB. The diluted mixture has a dimensional threshold of colorcorresponding to point 12, being about 1800 millimicrons.

Parts to be inspected are dipped in a tank containing theabove-described diluted penetrant. Following a dwell time of 10 minutes,the parts are removed from the tank and allowed to drain and dry. Duringthe drain-dry period of about 15 minutes, the water evaporates from thethin film of penetrant mixture on the parts and the sensitivity of theliquid film is restored to its original value of 500 millimicrons.

In this instance, the undiluted penetrant might cost $6.00 per gallon,while the diluted mixture costs only about $.60 per gallon, a costsaving for the tank mixture of about 90%.

In both of the above examples, a combination of method steps is employedwherein a penetrant of predetermined sensitivity is diluted with arelatively volatile diluent, and then test parts which have been dippedin the diluted penetrant are exposed in a drain-dry step such that thevolatile diluent is allowed to evaporate from films of diluted penetrantapplied to parts being inspected. An essential requirement for theproper performance of this method step is that the penetrant material,in its undiluted form, must be relatively non-volatile, so that it doesnot evaporae along with the volatile diluent.

The examples given are representative only, and the method step may beapplied to any type of penetrant material so long as the step ofevaporation of the volatile solvent does not carry withit a change inthe characteristics or chemical balance of ingredients in the undilutedform of the penetrant. Even those penetrants which contain relativelyvolatile ingredients may be employed with the above-described methodstep, provided the volatility of the diluent material is substantiallygreater, and pro-v vided that azeotropic mixtures are not formed whichcause a deviation in the sensitivity characteristic. A V

In the event that the undiluted penetrant contains in- V gredients whichform azeotropic mixtures with the diluent material, and there arethousands of such combinations known to chemistry, then it is, ofcourse, obvious that the initial undiluted penetrant could be formulatedwith an appropriate amount of diluent to provide an azeotropic mixtureto start with. Then, by building adesired dimensional sensitivity intothis azeotropic composition, dilution and subsequent evaporationof'diluent may be carried out as described above, yielding the desiredresult of restoration of sensitivity.

I claim:

1. In a penetrant inspection process employing a relatively nonvolatileliquid inspection penetrant containing at least one sensitizer dye, amethod for changing and restoring the sensitivity characteristic of saidinspection penetrant to its original value, comprising the steps ofdiluting said liquid inspection penetrant with a relatively volatilediluent, coating the surface to be inspected with a film of said dilutedpenetrant liquid, and drying said film to remove excess volatile diluentfrom said film.

2. The method steps in an inspection penetrant process in accordancewith claim 1 in which said step of diluting said liquid inspectionpenetrant with a relatively volatile diluent is carried out during theinitial formulation of said relatively nonvolatile liquid inspectionpenetrant.

3. A method in accordance with claim 1 in which said penetrant is afluorescent oily penetrant and said relatively volatile diluent ischlorinated hydrocarbon.

4. A method in accordance with claim 1 in which said penetrant is awater-miscible visible dye penetrant and said relatively volatilediluent is water.

5. The method steps in an inspection penetrant process in accordancewith claim 1 in which said penetrant is a water-miscible fluorescentpenetrant and said relatively volatile diluent is water. I

6. The method steps in an inspection penetrant process in accordancewith claim 1 in which said penetrant is an oily visible dye penetrantand said relatively volatile diluent is a chlorinated hydrocarbon.

7. The method steps in an inspection penetrant process in accordancewith claim 1 in which said penetrant is compatible with water and saidrelatively volatile diluent is water.

8. The method steps in an inspection penetrant process in accordancewith claim 1 in which said penetrant is compatible with chlorinatedhydrocarbons and said diluent is a chlorinated hydrocarbon.

9. The method steps in an inspection penetrant process in accordancewith claim 1 in which at least one of said sensitizer dyes is a visiblecolor dye.

10. The method steps in an inspection penetrant process in accordancewith claim 1 in which at least one of said sensitizer dyes is afluorescent dye.

References Cited by the Examiner UNITED STATES PATENTS 2,953,530 9/1960Switzer 250 71 x 3,028,338 4/1962 Parker 25071X 3,083,297 3/1963Lockwood 250 71 3,215,839 11/1965 Anderson 250-10 RALPH G. NILSON,Primary Examiner. WALTER STOLWEIN, Examiner.

ELBAUM, Assistant Examiner.

1. IN A PENETRANT INSPECTION PROCESS EMPLOYING A RELATIVELY NONVOLATILELIQUID INSPECTION PENETRANT CONTAINING AT LEAST ONE SENSITIZER DYE, AMETHOD FOR CHANGING AND RESTORING THE SENSITIVITY CHARACTERISTIC OF SAIDINSPECTION PENETRANT TO ITS ORIGINAL VALUE, COMPRISING THE STEPS OFDILUTING SAID LIQUID INSPECTION PENETRANT WITH A RELATIVELY VOLATILEDILUENT, COATING THE SURFACE TO BE INSPECTED WITH A FILM OF SAID DILUTEDPENETRANT LIQUID, AN DRYING SAID FILM TO REMOVE EXCESS VOLATILE DILUENTFROM SAID FILM.